1. Field of the Invention:
The present invention is related to a method for the separation and purification of methacrylic acid from an aqueous solution of methacrylic acid by a combination of extraction and distillation steps.
2. Description of the Prior Art:
Methacrylic acid may be produced by catalytic oxidation of isobutylene, tertiary butyl alcohol or methacrolein in the vapor phase, and is usually recovered from the reaction gas as an aqueous solution which also contains by-product acetic acid.
Separation of methacrylic acid from this aqueous solution by distillation alone requires many distillation operations, which greatly increases the cost of its recovery and purification. The difficulties of separating methacrylic acid-water systems, acetic acid-water systems, and methacrylic acid-acetic acid systems, respectively, are the cause of this problem.
Therefore, extraction methods have thus far been preferred as the means to separate methacrylic acid from its aqueous solutions, and many extraction solvents have been suggested.
It is well known that esters such as methyl methacrylate are favorable extraction solvents because of their high distribution coefficients ##EQU1## FOR METHACRYLIC ACID. However, when acetic acid is also present, it is extracted concurrently with methacrylic acid because the distribution coefficient of acetic acid is high, too. Further disadvantages are that the esters dissolve to a considerable extent in aqueous solution because of their high mutual solubility with water, and that as methacrylic acid is extracted into the esters, the solubility of water in the organic phase also increases.
To avoid these problems, many methods have been suggested, in which a mixed solvent is used containing the esters and hydrocarbons having low mutual solubility with water. These include a method using a mixed solvent of acetic or acrylic ester and toluene for the extraction of acrylic acid from an aqueous solution of acrylic acid (Japanes Pat. No. Sho 41-15569), a method using a mixed solvent of methyl methacrylate and xylene or ethylbenzene (Japanese Pat. No. Sho 49-41413), and a method using a mixed solvent of esters and aliphatic or alicyclic hydrocarbons (Japanese Pat. No. Sho 50-11364).
However, when acetic acid esters or acrylic acid esters are used for the extraction of methacrylic acid, they are slightly hydrolyzed and therefore in the subsequent process, the separation and purification of methacrylic acid from solvent becomes difficult, and in addition, the extractability of methacrylic acid is low. The choice of xylene or ethylbenzene as the hydrocarbon component of a mixed solvent, has the disadvantage that, due to the high boiling point of the solvent, methacrylic acid is easily polymerized by heating when the solvent is to be separated. Being highly polymerizable, methacrylic acid polymerizes to some extent even in the presence of a polymerization inhibitor if it is heated above 90.degree. C., thus making it less suitable for industrial application. Moreover, if the temperature is raised to about the boiling point of xylene or ethylbenzene, methacrylic acid is even more easily polymerized, further diminishing its industrial usefullness. Alicyclic or aliphatic hydrocarbons such as cyclohexane, methylcyclohexane and n-heptane are low in extractability of methacrylic acid, as shown in FIG. 1. Also, when the boiling point of the solvent is too low, separation from methacrylic acid is good, but a great deal of refrigerant is needed for complete recovery of the solvent. Thus, it has been found that in order to avoid heating methacrylic acid above more than 90.degree. C., and to run the solvent-separating process economically after extraction, the solvents to be used must be limited to an extremely narrow boiling point range.
A need, therefore, continues to exist for a method for extracting methacrylic acid from an aqueous solution which also contains acetic acid, wherein the solvent system used has an appropriate boiling point range and a high distribution coefficient while avoiding the solubility problems discussed earlier.